Staff: Mentor

Re: Japan Earthquake

Unless one has direct contact from someone in the plant, the best one can do is the releases by TEPCO or METI, but METI is probably getting it second hand. I haven't seen any official releases from METI though.

Staff: Mentor

Re: Japan Earthquake

At the moment, I don't see a 'real' chance of a meltdown. It is a worst case scenario, which is what licensing space is all about.

Some of the worst case scenarios involve extraordinary, and not necessarily physical real situations. It's a bit like crash testing a car by dropping it out of an airplane at several thousand feet, knowing full well that such a car would never be able to achieve such a velocity on level ground since the engine could not achieve the speed or the tires would blow out well before the speed could be achieved.

Some in the media will sensationalize the situation in order to grab the audience in order to sell commercials for useless stuff. But I digress.

I'd like to keep the discussion here on the technical aspects - as soon as we learn from reliable sources.

I'd like to keep the discussion here on the technical aspects - as soon as we learn from reliable sources.

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Ok. This question is pretty 'elementary' but I do not know much about how a NPP works. I read an article that said that the US delivered coolant to Japan. Why does Japan need the coolant? I don't understand how an earthquake and a tsunami can make a NPP lose its coolant? Also, what specific purpose does the coolant serve in the plant (what exactly does it cool) and what would happen if Japan could not get more coolant?

Staff: Mentor

Ok. This question is pretty 'elementary' but I do not know much about how a NPP works. I read an article that said that the US delivered coolant to Japan. Why does Japan need the coolant? I don't understand how an earthquake and a tsunami can make a NPP lose its coolant? Also, what specific purpose does the coolant serve in the plant (what exactly does it cool) and what would happen if Japan could not get more coolant?

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There was some miscommunication. I'm not sure who said what to whom, but apparently Sec of State Clinton mentioned coolant was being flown to Japan. That is incorrect.

They might airlift some generators, but I have not heard any details.

A light water reactor (LWR) uses water coolant under pressure to both moderate the neutrons and cool the nuclear fuel. The Fukushima units are boiling water reactors (BWRs), in which the water is boiled in the core at a pressure of ~1055 psia at Tsat ~ 286 C. The steam is sent directly to a high pressure turbine, and then the exhaust of the high pressure turbine feeds a set of low pressure turbines. The units are about 33% efficient, so about 67% of the heat is rejected through condensers to the environment, which at Fukushima is the sea.

When the reactor shutdowns, there is still some heat being generated from the decaying fission products. "At the moment of reactor shutdown, decay heat will be about 7% of the previous core power if the reactor has had a long and steady power history. About 1 hour after shutdown, the decay heat will be about 1.5% of the previous core power. After a day, the decay heat falls to 0.4%, and after a week it will be only 0.2%." Ref: http://en.wikipedia.org/wiki/Decay_heat#Power_reactors_in_shutdown

So it is important to cool the reactor for several days following shutdown.

The reactor core isolation cooling (RCIC) system in a boiling water reactor (BWR) is a safety system which serves as a standby source of cooling water to provide a limited decay heat removal capability whenever the main feedwater system is isolated from the reactor vessel.

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The RCIC requires power from off-site or emergency diesel generators. Well the plant lost off-site power. The EDGs worked for about 1 hour then stopped. Now plant personnel are trying to get power from some other source. It's not clear why the EDGs quit, but they are supposed to work for days.

The reactor is off but keeps producing heat has radioactive isotopes decay. Basically when it is on it produces a heat and radioactive isotopes which decay to more stable isotopes and give off more heat in the process. You can't turn this second process off, it just keeps going. The amount of heat the second process gives off goes down over time. However that means you have to keep cooling it for a number of days otherwise it would melt into a pile of radioactive goo (and take your expensive reactor with it).

It looks like the containment building has exploded. My educated guess is, what they feared might happen at TMI has happened here. The core melted down, releasing lots of hydrogen gas in the chemical reaction with the cladding, which then ignited.

Tepco has said that the pressure within the containment of Fukushima Daiichi 1 has reached around 840 kPa, compared to reference levels of 400 kPa.

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So pressure in containment is about 8.4 atm rather the 4 atm, the nominal design pressure.

and

Television cameras trained on the plant caputured a dramatic explosion surrounding unit 1 at around 6pm. Amid a visible pressure release and a cloud of dust it was not possible to know the extent of the damage. The external building structure does not act as the containment, which is an airtight engineered boundary within. The status of the containment is not yet known.

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and meanwhile at the plant next door, Fukushima Daiini (plant 2)

Unit 1's reactor core isolation cooling system had been operating normally, and this was later supplemented by a separate make-up water condensate system. However, the latter was lost at 5.32am local time when its suppression chamber reached 100ºC. This led Tepco to notify government of another technical emergency situation.

However, I understand that the building where the explosion has occurred is not associated with containment, but I have not been able to verify this.

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A TEPCO spokesperson made a statement on Japanese TV about 20 minutes ago. The explosion was supposedly from a buildup of hydrogen around the containment vessel. Apparently, they will flood the containment vessel with seawater (and boric acid?) Does that make sense? They also increased the evacuation zone to 20km. The reporters present didn't seem to like the contradiction of TEPCO's claim that everything was still safe and the expansion of the evacuation zone.

Caveat: my Japanese is not so great, so I may have misunderstood some of it.

EDIT: after checking online, looks like I got most of it right, but it was a government official, not a TEPCO spokesperson

I have to say I don't understand how you can have a hydrogen explosion blowing apart the confinement building, and not the reactor vessel.

I also don't understand how you can let any pressure build up in the confinement building at the risk of rupture if it is slowly. One should prefer steam releases (even contaminated) in order to ensure the integrity of the confinement building if ever the reactor vessel breaks, no ? Now we are not very far from a full release of the core in the environment.

Staff: Mentor

A TEPCO spokesperson made a statement on Japanese TV about 20 minutes ago. The explosion was supposedly from a buildup of hydrogen around the containment vessel. Apparently, they will flood the containment vessel with seawater (and boric acid?) Does that make sense? They also increased the evacuation zone to 20km. The reporters present didn't seem to like the contradiction of TEPCO's claim that everything was still safe and the expansion of the evacuation zone.

Caveat: my Japanese is not so great, so I may have misunderstood some of it.

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I have read much the same in English. So far, I have heard conflicting claims - that the explosion was / was not the containment building. I'd have to find a map of the site to figure out if the explosion was near unit 1 or 4. Unit 1 has the most severe problem, while unit 4 was shutdown cold already. From the picture, it looked like the explosion was near unit 4.

The articles are just popping up on Japanese pages. The latest may not be translated to English yet. Every Japanese channel has been showing explanations about the containment vessel and the seawater insertion, so I guess they are going to do it.

More than like the explosion was within the Off-gas system external to the reactor building and below the off-gas vent towers you see. Primarily due to radiolytic hydrogen build-up. Radiation level increases would be due to premature release of radioactive noble gases (Ze, K, I whcih are normal held up for decay).

The suppression chamber at 100 C appears to be within or at the limit of DBA parimeters. It is doing what it is supposed to do cool the water and provide cooling water to the LPCIS. The steam driven HPISs are probably out due to lack of vaible carry over steam in the system. If the diesel generators went south then the LPCIS will be failling.